Hi. I'm trying to get the frustum vertices by using the perspective projection matrix's inverse. I'm trying to take a cube with the vertices { (-1,-1,-1), (1,-1,-1) ... } and multiply each vertex by the inverse of the perspective matrix, hoping to get the frustum. This isn't working, however. I am aware that there's a lighthouse3d tutorial, and I just might bite the bullet and go through it, but I'm just curious why this isn't working.

The projection matrix uses a projective space with homogeneous coordinates. This means that you have a 4th component. To get a vector from your normal 3 dimensional space to this projective space you attach a 4th component and set it to 1. In your case it is 0 for some vectors which is incorrect.
After the transformation with the inverse projection matrix the homogeneous vector must be transformed back from projective space to your normal 3 dimensional space:
tempVec /= tempVec.w

Also note that OpenGL returns the matrix column major IIRC and glm _might_ store it row major (I have never used glm). In this case you would have to transpose the matrix.